In recent years, a variety of flexible plastic membranes have been developed and marketed as alternatives to glass for many glazing applications such as greenhouses, windows, skylights, and as covers for solar-energy collection systems. These membranes can have high transmissivity to light, are more resistant to breakage, and are lighter and often less costly than glass. However, when such membranes are used exposed to the exterior of buildings, their characteristic lack of plate strength and low modulus of elasticity have imposed difficulties in stabilizing them in order to inhibit random wrinkling and sag, to limit stress from snow and other environmental loads, and to prevent flutter from wind action, all of which can adversely affect their market acceptability and service life.
In typical applications of smaller scale where governing design considerations require the installation of a glazing membrane within a flat planar quadrilateral glazing frame, the membrane is usually held taut by an edge-framing system which maintains the membrane in planar tension between one or both pairs of opposing frame members. To operate satisfactorily over a long-term service life, the frame must continuously adjust to sustain membrane tension during differential dimensional changes in both itself and the membrane caused by variations in temperature and moisture, as well as during permanent creep and enlargement of the membrane caused by the tensioning stress. Due to difficulties in meeting this requirement, many systems for tensioning membranes in planar frames are diminished in usefulness by complexity of detail, high cost of manufacture, difficulty in on-site assembly or repair, or by inability to provide the desired uniform tension over long service.
As an alternate to planar tensioning, membranes may be held and tensioned within edge framing assemblies which are non-planar and thereby warp the membrane into various types of opposing compound curvatures. Examples of warped surfaces with such curvatures may be found on saddles, hyperbolic parabaloids, and in the configurations of many contemporary tent designs.
For a given uniform load normal to the membrane, warped membranes tend to react to the loading with lower internal stress, and therefore lower deflection, than do planar membranes of equivalent material and area. Because resistance to deflection under load is often the most significant criteria of stability, warped membranes will require less tension and therefore will impose lower bending stress on the frame, for a given degree of stability, than will planar membranes which are otherwise equivalent. Therefore, techniques which provide membrane stabilization by means of such warped, non-planar, tent-like configurations have proven highly superior to techniques for stabilization which employ planar configurations, and thus warped membranes have been repeatedly chosen and successfully used to cover or enclose expansive portions of numerous large buildings.
However, despite the superior stabilizing ability of the warped configurations, in applications of reduced scale where smaller membrane areas of less than approximately 200 square feet are held within a single peripheral framing system, the use of warped surfaces has been inhibited, partly due to a commonly held presumption that a non-planar peripheral frame or other non-planar supporting structure is necessary to achieve the desired warp. Due to its atypical corner joinery, such a frame is often difficult and costly to achieve in a manner compatible with the rectilinear subassemblies which are common in ordinary low-cost construction.
From the preceding brief analysis, it becomes evident that the use of exterior membrane glazing in small areas has been limited in part by an apparent incompatibility between the higher stability available from warped membrane surfaces and the convenience of planar edge-framing conditions.